1. Field of the Invention
This invention relates to a bypass isolation open or closed transition automatic transfer switch assembly, commonly called an automatic transfer switch assembly (ATS assembly) and, more specifically, an ATS assembly having a latch assembly structured to resist, and preferably prevent, actuating the switch during a high fault current.
2. Background Information
Certain installations, e.g. hospitals, (hereinafter “the system load”) must have power systems structured to provide an uninterruptable power supply. The primary power source is typically the public power grid and the secondary power source is typically a generator. Both of these sources are structured to provide power over an extended period of time. That is, the system typically draws power from the primary power source, however, if that source becomes disabled for any period of time, the secondary source is used. Generally, it is not desirable to have two power sources providing electricity at the same time. That is, the currents from the sources are typically in different phases and cannot be combined safely.
The ATS is not a circuit breaker or similar device structured to interrupt the current in the event of an over-current condition. Circuit breakers, and similar devices, structured to protect the system load from over-current conditions are typically located upstream and/or downstream of the ATS.
The ATS includes a housing, an operating mechanism, a first line bus, a second line bus, a load bus, a first line movable contact, a second line movable contact, a fixed contact assembly, and a control device. The operating mechanism, first line movable contact, second line movable contact, fixed contact assembly, and control device are disposed within the housing. The first line bus is substantially disposed within the housing but includes a terminal that extends outside the housing. The first line bus terminal is coupled to, and in electrical communication with, the primary power source. Similarly, the second line bus is substantially disposed within the housing but includes a terminal that extends outside the housing. The second line bus terminal is coupled to, and in electrical communication with, the secondary power source. The load bus also is disposed, substantially, within the housing and includes a terminal that extends outside the housing. The load bus terminal is coupled to, and in electrical communication with, the system load. The fixed contact assembly is coupled to, and in electrical communication with, the load bus. The fixed contact assembly is structured to be engaged, alternately, by the first line movable contact and the second line movable contact.
The first line movable contact is coupled to, and in electrical communication with, the first line bus. The first line movable contact is structured to move between a first position, wherein the first line movable contact does not engage the fixed contact assembly, and a second position, wherein the first line movable contact engages, and is in electrical communication with, the fixed contact assembly. Similarly, The second line movable contact is coupled to, and in electrical communication with, the second line bus. The second line movable contact is structured to move between a first position, wherein the first line movable contact does not engage the fixed contact assembly, and a second position, wherein the second line movable contact engages, and is in electrical communication with, the fixed contact assembly. Only one of the first and second movable contacts engages the fixed contact assembly at a time. That is, in the normal operating configuration, the first contact assembly is in the second position, thereby providing electricity to the system load from the primary power source, and the second contact assembly is in the first position. If the need arises, the first contact assembly is moved into the first position while the second contact assembly moves into the second position. The transfer occurs almost instantaneously. In this configuration, the secondary power source provides electricity to the system load.
Operation, i.e. positioning of, the first and second contact assemblies is performed by the operating mechanism. The operating mechanism includes a plurality of mechanical linkages that are configured to ensure that both the first and second contact assemblies are not in the second position at the same time. Both the first and second contact assemblies may be in the first position at the same time, i.e. the system load would not be receiving power from the ATS. The operating mechanism includes one or more springs structured to maintain the engaged contact assembly in the second configuration.
The control device actuates the operating mechanism. The control device, typically a solenoid, is structured to receive a control signal from a remote, or local, location. Thus, when the control device receives the control signal, the operating mechanism is actuated and the first and second contact assemblies move into a different position.
An ATS must be tested to ensure the ATS meets a defined operating criteria. As part of the testing procedures, the ATS is subjected to a “withstand” current. This is, essentially, an intense and sudden over-current. When such a current passes through the fixed contact assembly a strong magnetic field is created. This magnetic field may be strong enough to overcome the bias of the operating mechanism springs maintaining the engaged contact assembly in the second position. That is, the magnetic field causes the closed contact to separate from the fixed contact. If the over-current is sufficiently strong, the closed contact is rapidly moved into the first position. This, in turn, causes the operating mechanism to be actuated and move the other contact assembly into the second position. That is, the withstand current may cause the ATS to switch from one power source to the other. If this occurs, the ATS has failed the test.